On the planet Shan Shen, Donna meets a mysterious fortune teller. The woman persuades Donna to reveal the event which led to her original meeting with the Doctor -- and then Donna's world suddenly changes, as that crucial choice is undone. Now Donna Noble lives in a world without the Doctor: a world in which London is destroyed by the spaceship Titanic, America is devastated by the Adipose, and the entire planet is nearly annihilated by the Sontarans. Only an enigmatic blonde traveller from a parallel universe can help Donna restore the course of history, and prepare her to face the oncoming darkness.

The story that would become Turn Left was conceived on February 26th, 2007, when Doctor Who executive producer Russell T Davies was travelling to London by train. It was already planned that Season Thirty would include one story shining the spotlight squarely on the Doctor but barely featuring his companion, and a second story which would reverse these roles. This would enable the two episodes to be made alongside one another, as had become necessary in recent years in order to accommodate the production of a Christmas special in addition to the regular run. Previously, the double-banking had been facilitated through the inclusion of an adventure which relied heavily on a character other than the Doctor and his companion, but Davies wanted to try a different approach this time. He needed a reason to exclude the Doctor from the companion-centric narrative, and he hit upon the notion of an alternative timeline in which the Doctor had been killed because of a seemingly incidental change to his companion's personal history. Davies' inspiration was the 1998 Gwyneth Paltrow film Sliding Doors, which simultaneously explored the consequences of a woman's success and failure in boarding a train.

At this stage, the Doctor was to be accompanied throughout Season Thirty by a new character called Penny. In her debut adventure, Penny and her mother would be travelling to visit Penny's grandfather when they became trapped under a giant dome of alien origin. To tie in with his ideas for the companion-centric episode, Davies now decided that an early scene would see Penny faced with the innocuous choice of turning left or right at a T-junction. She turned left -- which resulted in her being trapped under the dome, meeting the Doctor, and ultimately saving his life. But in Turn Left, an alien creature would alter history so that Penny turned right instead -- meaning that she wound up on the outside of the dome, and the Doctor perished. Turn Left would then revisit Season Thirty's other modern-day stories, depicting a world increasingly devastated by the Doctor's absence while allowing the episode to be something of a budget-saver.

Turn Left would see the full-scale return of the Tenth Doctor's original companion, Rose Tyler

In addition, following some brief teases earlier in the season, Turn Left would see the full-scale return of the Tenth Doctor's original companion, Rose Tyler, as played by Billie Piper. She had been written out of Doctor Who in Doomsday, the Season Twenty-Eight finale, after becoming trapped -- seemingly forever -- in a parallel universe. But even back then, Piper and Davies had agreed that Rose would return at some point in the future, and Davies had been careful to seed Season Twenty-Nine with occasional reminders of her fate, to ensure that she was not forgotten by viewers. Finally, it was agreed that Piper would return for the final three episodes of Season Thirty, which would be the last full year of Doctor Who to be overseen by Davies.

In mid-March, Davies' plans for Turn Left changed considerably when Catherine Tate agreed to rejoin Doctor Who as Donna Noble, the character she had played in the 2006 Christmas special, The Runaway Bride. As a result, plans to introduce Penny Carter were immediately abandoned. Although the fact that Donna was an established character suggested that the “turn left/right” moment could no longer appear in the Season Thirty premiere, Davies was enthusiastic because it meant that he could now explore Donna as she was when she was first introduced: shallow, self-centred and ignorant. Over the coming months, Davies decided that the pivotal moment at the heart of Turn Left would now be Donna's decision whether or not to travel to her interview with HC Clements, her employer in The Runaway Bride.

The replacement of Penny with Donna also meant that Davies could incorporate even more stories into the revised chronology of Turn Left, since the Doctor would now die after flooding the lair of the Empress of the Racnoss in The Runaway Bride. Davies considered exploring how stories set in the past or on other planets would be affected by the Doctor's demise. For instance, he thought that the Unified Intelligence Taskforce (UNIT) might send “time commandos” to the year 1599 to foil the plot of the Carrionites, whom the Doctor had defeated in Season Twenty-Nine's The Shakespeare Code. Ultimately, however, he decided that this would bog down the episode in minutiae, which was particularly undesirable in a story which might be hostile to casual viewers.

Davies began writing his script on October 27th. Around this time, he dropped a subplot in which Donna got married and had children. It was intended to add even more drama to the character's decision to sacrifice herself and unravel the parallel timeline: not only would she be giving up her own life, but her offspring would be erased from history. However, Davies had now become aware that Steven Moffat had similar plans for Donna in Silence In The Library / Forest Of The Dead -- which, at this point, was intended to immediately precede Turn Left -- and he agreed that it was more pertinent to Moffat's narrative than his own.

Russell T Davies connected the Time Beetle to the Trickster, the history-bending villain introduced in The Sarah Jane Adventures

Davies completed his first draft of Turn Left on November 2nd. In addition to The Runaway Bride, the adventure revisited several other stories: the Season Twenty-Nine premiere, Smith And Jones, with the protagonists of the spin-off series The Sarah Jane Adventures and erstwhile companion Martha Jones all perishing when the Judoon transported Royal Hope Hospital to the Moon; the 2007 Christmas special, Voyage Of The Damned, with the spaceship Titanic now crashing into Buckingham Palace and exploding, thereby decimating southern England; the opening episode of Season Thirty, Partners In Crime, with the Adipose killing millions in the United States; and episodes four and five, The Sontaran Stratagem / The Poison Sky, with Gwen Cooper and Ianto Jones from Doctor Who's other spin-off, Torchwood, sacrificing their lives to stop the Sontarans, while their leader, Captain Jack Harkness, was taken to Sontar as a captive. Davies also decided to connect the Time Beetle at the heart of his narrative to the Trickster, the history-bending villain introduced by Gareth Roberts in Whatever Happened To Sarah Jane?, a 2007 serial for The Sarah Jane Adventures.

Turn Left was scheduled to be made on its own as Season Thirty's seventh recording block. It would be filmed in parallel with the Doctor-centric Midnight, which formed part of Block Six. Because Turn Left was a double-banked production, it was decided that Susie Liggat would once again act as producer in place of Phil Collinson; as usual under such circumstances, Collinson would instead be credited as an executive producer. A similar arrangement had been employed for the previous year's double-banked stories -- Human Nature / The Family Of Blood and Blink -- since when Liggat had produced three episodes during Season Thirty. Appointed to direct Turn Left was Graeme Harper, who had recently completed work on Block Two, comprising Planet Of The Ood and The Unicorn And The Wasp.

Most of Turn Left was filmed in Cardiff, starting on November 22nd with the footage of the news anchors at BBC Broadcasting House. The main shoot then began on the 26th, when scenes in Jival Chowdry's business and at the housing office were recorded at Bay Chambers. The same day, Billie Piper returned to Doctor Who after a twenty-month absence when the sequence of UNIT taking away the Doctor's body was captured on Hamadryad Road and Hunter Street. The BBC took the opportunity to officially confirm that Piper would be coming back to Doctor Who for three episodes.

Work on November 27th began on Clearwater Way, which was the terminus of Donna's time-jump. Filming then shifted to a residence on Nant-Fawr Road, which had regularly appeared as the Nobles' home throughout the season. On the 28th, Donna's fateful decision at the T-junction took her from Court Road onto Heol Gabriel, while the Lady Mary Allotments in Roath Park were the venue for Wilf's stargazing in Leeds. November 29th saw Donna sacrifice herself on St Isan Road, and Rose warned Donna to leave London for Christmas on Franklen Road. November 30th took Harper's team to Doctor Who's usual studio home in Upper Boat, for scenes in both the TARDIS and the fortune teller's parlour. The Time Beetle, which debuted on this day, had been intentionally designed to be reminiscent of the giant spider which affixed itself to Sarah Jane Smith in 1974's Planet Of The Spiders.

A hectic December 1st marked David Tennant's only full day of recording for Turn Left. First, the Shan Shen marketplace was erected at The Maltings, with a number of Asian extras having been hired through the Facebook social media site. Unfortunately, a miscommunication had led them to believe that their fee would be ten times higher than was actually being offered, and many left as the morning wore on. With the location situated very close to the Cardiff Royal Infirmary, Harper also took the opportunity to tape the news item regarding Royal Hope Hospital there. For this material, Ben Righton reprised his role as Oliver Morgenstern from Smith And Jones. Finally, cast and crew returned to Upper Boat for more sequences in the fortune teller's lair.

Catherine Tate came down with the flu on December 6th so recording proceeded at Sophia Gardens, the production unit base

Following a day off on the Sunday, the production ventured outside Cardiff to Porthkerry on December 3rd, with the Egerton Grey Country House hosting the Nobles' Christmas getaway, and footage of the coach captured along Port Road. On December 4th and 5th, the Nobles' lodgings in Leeds were actually a house on Machen Street in Penarth. Filming inside the dwelling took place on the first day, while exterior scenes were the focus of the second day, which also saw some material taped on the adjacent Rudry Street. It was back to Cardiff on December 6th, when The Conway was the bar at which Donna and her friends celebrated Christmas, and the pub patrons witnessed the Webstar attack from nearby Mortimer Road. That evening, Donna's conversation with Rose on the bench was supposed to be performed at Thompson Park. However, Tate had come down with the flu, and so it was decided that she would be more comfortable if recording instead proceeded at Sophia Gardens, which was being used as the production unit base. Principal photography concluded with two days -- December 7th and 8th -- at the disused Panteg Steelworks in Pontypool, for sequences in the UNIT warehouse. A number of inserts were also completed there on the latter day.

Into 2008, a small amount of work was still left to be done on Turn Left. Executive producer Julie Gardner wanted the closing seconds to feature more glimpses of the phrase “Bad Wolf” -- including on the TARDIS itself -- and so they were taped on January 18th at Sophia Gardens. Additional inserts were recorded at Upper Boat on the 24th. Jason Mohammad remounted some of his newsreader material at BBC Broadcasting House on January 31st. Finally, the concluding TARDIS scene was filmed at Upper Boat on March 20th.

During post-production on Turn Left, it became clear that the scale of the season's final three episodes was such that Doctor Who's regular forty-five-minute timeslot would be insufficient. The BBC instead agreed to allot fifty minutes to Turn Left, allowing Harper to complete the episode without having to leave large chunks of Davies' script on the cutting room floor.

Amidst a blaze of publicity for Rose's return, Turn Left was transmitted on June 21st. Its 6.40pm start time was earlier than in recent weeks, due to the possibility of BBC One broadcasting a UEFA Euro 2008 match later in the evening. When this did not come to pass, Doctor Who wound up leading into a repeat of Piper's turn as Philip Pullman heroine Sally Lockhart in The Ruby In The Smoke. Despite airing half an hour earlier, Turn Left attracted the same total audience of 8.1 million as Midnight had garnered seven days earlier. Indeed, Turn Left actually placed one spot higher in the weekly viewing charts, landing in fourth position. In all of Doctor Who's long history, only Voyage Of The Damned had done better, with a second-place showing at Christmas 2007. And Doctor Who's momentum was still building...

• Doctor Who Magazine Special Edition #20, 19th November 2008, “Episode 11: Turn Left” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #59, 2017, “Story 197: Turn Left”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Writer's Tale by Russell T Davies and Benjamin Cook (2008), BBC Books.